David De Gea then had to tip over a Yannick Bolasie header, before United regained control.
Wales keeper Wayne Hennessey made a fine save to deny Mata and Fellaini had a header deflected wide when he looked like scoring from a Daley Blind corner.
United deserved to take the lead in the 33rd minute, when brilliant play by Rashford led to Anthony Martial striking the ball towards goal, only for Joel Ward to divert it for a corner.
Wayne Rooney, in his deeper role, pulled the strings for United for long periods.
And with United generally on top, Fellaini crashed a right-foot drive against the post in the 53rd minute.
United’s luck was out again nine minutes later with Martial, this time, hitting the post with a fine header from Valencia’s cross.